Drago, Hendrickson, et al.
(19) (20) (21) (22)
/ p-Pyrazine-bis(pentaammineruthenium)Tosylate
P. K. Gantzel, R. A. Sparks, and K. N. Trueblood, UCLA L S ~ American , Crystallographic Association Program Library (old) No. 317 (revised 1976): Fourier program, C. R. Hubbard, C. 0. Quicksall, and R. A. Jacobson, Ames Laboratory Fast Fourier, Iowa State University, 1971; C. K. Johnson, ORTEP, Report No. ORNL-3794, Oak Ridge National Laboratory, Oak Ridge, Tenn., 1965. S. W.Peterson and H. A. Levy, Acta Crystallogr., I O , 70 (1957). P. E. Riley, K. B. Kunz, and K. Seff, J. Am. Chem. SOC., 97, 537 (1975). Y . Kim and K. Seff, J. Am. Chem. SOC., 100, 175 (1978). D.W.J. Cruickshank, Acta Crystallogr., 2, 65 (1949).
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(23) "International Tables for X-Ray Crystallography", Voi. IV. Kynoch Press, Birmingham, England, 1974, pp 73-87. (24) Reference 23, pp 149-150. (25) "International Distances, Supplement", Chem. Soc.,Spec. Pub/., No. 18, M2s (1965). (26) "Handbook of Chemistry and Physics", Chemical Rubber Publishing Co., Cleveland, Ohio, 1974, p D-157. (27) Reference 26, p F-201 (28) Y . Kim and K. Seff, J. Phys. Chem., 82, 1071 (1976). (29) Reference 26, p F-198.
Experimental Evidence for Trapped Valences in the Mixed-Valence Complex p-Pyrazine-bis(pentaammineruthenium) Tosylate. Electron Paramagnetic Resonance, Magnetic Susceptibility, and Nuclear Magnetic Resonance Results Bruce C. Bunker, Russell S. Drago,* David N. Hendrickson,* Robert M. Richman, and Stephen L. Kessell Contribution f r o m the W. A . Noyes Laboratory, Unicersity of Illinois, Urbana, Illinois 61801. Receiced October 21, 1977
Abstract: The results of the variable-temperature magnetic susceptibility, EPR, and N M R experiments on salts of [ ( ( N H3)y Ru)*pyrln+ ( n = 4, 5 , 6 ) and on a salt of [((XH3)5Ru)pyrl3+ are presented. The unpaired electron in the mixed valence [ 2 , 3 ] oxidation state of the dimer is localized in a (d,,, dy,) orbital on the Ru(ll1) center. The electron from the R u ( l l ) center is transferred through the pyrazine bridge to the other metal site at a rate which is fast on the N M R time scale, but slow on the E P R time scale. The energy barrier to the thermal electron transfer is calculated to be between 3.4 and 6.7 kcal/rnol.
Since Creutz and Taube' first reported the synthesis of [([NH3)5Ru)2pyrl5+, this mixed valence dimer has fostered a great deal of interest and controversy in the literature. Most of the controversy centers on the thermal electron transfer process postulated to occur between the two metal centers in the complex. Accurate rate data for the thermal electron transfer in this simple model system could provide insight into
4+
5.
the current theories of electron transfer phenomena. Although many workers have attempted to obtain such rate data for the thermal electron transfer process, the experimental conditions appropriate for observing the phenomenon as a dynamic process have not been found. The unpaired electron either appears to be completely localized or completely delocalized on the time scale of all of the experiments carried out to date, as shown in Table I . The conclusions of several of these experimental studies are based on subtle effects, making it difficult to decide whether the data indicate that the electron is localized or delocalized. Thus, it is not surprising that there is disagreement in the literature concerning the rate of thermal electron transfer. 0002-7863/78/1500-3805$01 .OO/O
There is also disagreement about the nature and magnitude of the energy barrier to thermal electron transfer in this interesting complex. Robin and DayI2 have distinguished three classes of mixed-valence complexes based on the extent of overlap between the metal orbitals containing the unpaired electron and the orbitals of the bridging ligand. A similar classification scheme is depicted in Figure 1. If the orbital interactions between the two metal centers through the bridge are negligible (class I), the potential barrier to thermal electron transfer can be defined by two harmonic oscillator potential wells. This vibronic barrier is predicted by the Hush theory" to have an energy which is one-fourth the energy of the intervalence transfer band observed i n the electronic spectrum of the complex at 1570 nm. If there are extensive metal-metal interactions (class HI),the orbital containing the unpaired electron could be delocalized over the entire complex. Here, electronic effects totally remove the vibronic barrier to the electron transfer. The optical and electronic properties of the pyrazine bridged dimer have led Taube and others to classify it as a class I1 complex described by a localized site molecular orbital scheme in which the vibronic barrier to the electron transfer is lowered to some extent by electronic effects but is not eliminated. Hush argues in favor of a class I11 designation based on the IR results mentioned in Table I and in light of the fact that the so-called intervalence transfer band shows neither the solvent dependence nor the bandwidth predicted for intervalence transfer transitions by the Hush theory. Clearly, there is confusion concerning both the molecular orbital description of the mixed valence pyrazine bridged dimer and the rate of thermal electron transfer between the two metal centers. In an attempt to further our understanding of both of these features of this interesting compound, we have carried out a series of EPR, N M R , and magnetic susceptibility ex-
0 1978 American Chemical Society
Journal of the American Chemical Society
3806
1
100:12
1 June 7. 1978
Table I. Electron Transfer Rates for [2,3]pyr
Techniaue
K TemD.
Sample state
300 200 I50 80 300 300 302 4 300
Solution Powder Powder Powder Powder Powder Sol 11 t ion Solution Powder KBr pellet
N e a r IR1.I1 (Hush) (Hush) (Hush)
(Hush) ESCA4 Raman6 NMR2 Mossbauer3
I ~7
EbJrricr
Rate.
3 x IO'" 4 x I 07 (1 7 x IO'(' 6 X IO-'
[2 31
t
I1
.2 3j
O < a
